For my system with the specs listed below, and the following two cards at their approximate price. Which card will be my best option for Oblivion. Since I was running a 5600Ultra before, either one of them would be a big upgrade.
6600GT for ~$100
6800GT for ~$225
AMD Athalon XP 3000+
1G Corsair memory
Personally I'd recommend you another card, but I'll stick with nVidia since you seem to prefer that route, the GF6800GS as was just mentioned is a great bang/buck, and IMO the minimum you want for good gameplay with Oblivion. Unfortunately it's not as fast as it's PCIe namesake though.
Just keep in mind that even a GF6800GS will require you to use very lose resolution. Download the latest 'Oblivion' drivers from nVidia first, the let the machine autodetect your settings and don't be surprised if even then you have trouble near the gates of oblivion themselves when you have many NPCs on the screen at the same time.
IMO the GF6600GT is too weak to truely enjoy Oblivion without major stuttering, the GF6800GS should be a great choice, the GF6800Gt if you can't get the GS.
If might be worth considering upgrading to a PCI-Express motherboard for $80 - $100 and a new PCI-Express VGA for $200 - $300 if you have the budget for it.
Yes, that is what I understood from reading the sticky thread on top too. The problem is that my CPU is Socket A and I am under the impression I can not get a PCI-E with Socket A. I don't really want to buy a new computer right now either, since I would rather wait for the new CPU's later in the year, since it looks like they will be using different motherboards.
I guess in the end the question probably always comes down to how much someone wants to spend.
My 9800np plays Oblivion great (at 800x600 and no AA/AF, but shhh).
Actually, it's damn annoying. I started at 1280x1024 with 6xAA (ie. max with the exception of HDR) and it was so lovely A slideshow, but lovely nonetheless. At 1024x768 2xAA everything worked and looked like a good balance, but every 3mins or so it would go to 1FPS for 10 seconds. To get round that, I had to go to the resolution above. Hell, Morrowind probably looked better.
(Rest of my system is 1024MB PC3200, P4 2.8C, i875 mobo)
Anyway, just thought you'd like to know. I'm kinda in the same position in you, with a guy wanting to sell me a 6800GT for £130, 7800GSs on sale for £200, or I could just get a new system for ~ £650. Hell, could even get an Xbox360 I suppose. But then, I'd rather not do anything just yet, until I see how Conroe/AM2 shape up, or how/if the 360 gets, ahem, liberated.
I had the same issue as you, I was running an AMD XP 2200+ and needed to upgrade from my 5700 Ultra...so I went with the 6800gt and I have never regretted it...I even stayed with it and upgraded to socket 939 and an X2 but stayed with AGP because the cost is just not worth it going to PCI-e for me. If you do decide to go PCI-e, wait till year end after M2 has come out, you could probably grab a PCI-e board SLI and some 7800gt's for 500 total...that's what im waiting for.